I watch some threads here but sometimes I don't get alerts when there is an update on the thread. Why?

[Unbanned User]

This happens randomly, Why?

 

[White_Knight]

This happens randomly, Why?

It most likely gets buried by other alerts (happens to be all the time). It will only alert you once and if you don't check it you won't get another alert from that thread unless you are directly quoted and/or someone has reacted to your post (again, this usually buries my watched thread updates and I miss it).